Caustic soda turned intto black slime...

[Karl D]

I recently obtained a cast metal ashtray that looks like it is cast in zinc. It'ss actually an admiralty ashtray from HMUDE (Her Majesty's Underwater Defence Establishment. It had some stains inside it so I sprinkled some caustic soda crystals in there (since they just happenned to be at hand) and topped up with water. 30 minutes later, I was astonished to find the thing full of black slime! Some sort of chemical reaction, I'm guessing.... but what? Anyone here know enough chemistry to tell me how/what/why??

Thanks - I'm just curious....!

K

[Andrew Gabriel]

Zinc is amphoteric, which means it will dissolve in both acids and alkalis. If it is zinc, then you created sodium zincate. If it is aluminium (or an aluminium alloy -- some look like zinc), that is also amphoteric.

[The Natural Philosopher]

Mezac probably is the metal, and that would be summat like zinc hydrozde or oxide perhaps.

Use an acid to cleam it of. Star with vinegare, work our way up thrugh decsalers and bolied rhubarb leaves to brck acid.

[Michael Mcneil]

I think it will be zinc with about 4% aluminium which as we all know is added to the mix in order to reduce the affect of molten zinc on the steel molds used in its casting.

How nice to fit another caustic soda thread in before sickmess.

Soda diz gerizzmerzz Happy new ear To all the people living in fear.

[John Rumm]

Think I will have a pint of what he is drinking!

[Marcus Fox]

Zinc will dissolve in hydroxide (alkali) to form zincates [Zn(OH)4]2- and [Zn(OH)3]-. It is amphoteric, and will also dissolve in acid, to release hydrogen gas.

Marcus

[raden]

Is that the star in the East ... again ?

[Marcus Fox]

No. Zinc dissolves in acid and releases hydrogen gas.

Marcus

[The Natural Philosopher]

Yes, but slower thn the hydroxides and zincates will.

The idea is to clear rhe sludge and etch teh actual metal a bit to brighten it up.

Ive used this trick on brass. Works well.